Transpiration cooled induction plasma generator



H. U. ECKERT Sept. 19, 1967 TRANSPIRATION CUOLED INDUCTION PLASMA GENERATOR Filed March 16, 1965 INVENTOR. HANS U. ECKERT Acjent United States Patent 3,343,022 TRANSPIRATION COOLED INDUCTION PLASMA GENERATOR Hans U. Eckert, Spring Valley, Calif., assignor to Lockheed Aircraft Corporation, Burbank, Calif. Filed Mar. 16, 1965, Ser. No. 440,256 6 Claims. (Cl. 313-63) This invention relates to a method and apparatus for generating a plasma stream or plasma jet, and more particularly relates to a transpiration cooled, inductive plasma generator for producing a constricted-gas-stabilized electrodeless discharge in a streaming gas.

Various types of apparatus for producing plasma jets have been constructed heretofore. Generally, these devices produce a high-current electrical arc between two electrodes in a small chamber. A coolant such as Water or a noble-gas, injected into the chamber to cool it, makes the arc hotter by inducing two effects. The cooling makes the outer region of the arc less conductive and so, by a thermal pinch effect the current density in the center of the discharge is increased. A magnetohydrodynamic pinch effect then increases the current density further, bringing the plasma to an extreme temperature. Magnetohydrodynamic forces and pressure eject the plasma into the atmosphere through an orifice.

The noble-gas plasma offers pure heat at temperatures up to 30,000 F. This temperature exceeds not only the melting point, but also the boiling point, of all the elements.

A typical prior device employs a small cylindrical chamber made of glass or metal in which an arc is struck. The front end of the chamber is provided with a negative electrode comprising a flat plate of carbon or other conducting material. This electrode has an aperture at its center to provide an orifice through which the plasma jet is ejected. The opposite end of the chamber is closed and is provided with an insulated positive electrode. DC power is applied between the electrodes. Cooling is provided by a jet of water or gas injected tangentially into the chamber. The are is contained within a fluid sheath formed by the high pressure steam or gas which forms between the arc and the walls of the chamber. The main advantage of this method is that DC power can be furnished with comparatively little expenditure. A disadvantage of this prior art device is that the plasma is in contact with the electrodes and at desirable power densities electrode erosion cannot be avoided, notwithstanding a high rate of water cooling. This results in undesirable changes in the operational characteristics of the arc, and in many cases also results in undesirable flow contamination. These shortcomings are overcome by the method and apparatus of the present invention. Additional advantages of the present invention are that water cooling is eliminated altogether, and higher electron concentrations and thus high conductivities are achievable.

A purpose of :the present invention is to produce clean and stable plasma jets for the investigation of high temperature and magnetogasdynarnic phenomena. Such apparatus is useful for simulation of hypersonic re-entry conditions, chemical analysis and synthesis, melting of refractory materials, and welding.

In certain prior art devices, the induction principle has been employed rather than a direct current arc, in an effort to overcome certain of the previously discussed shortcomings of prior devices. In one such device the gas is ducted through the discharge zone via a cylindrical quartz tube. Since a considerable portion of the ionization energy is converted into heat at the tube wall, cooling must be employed. The heat flux which can be conducted across the walls at structurally safe temperatures places an upper limit on the possible power input. Also,

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3,343,022 Patented Sept. 19, 1967 the coolant carries away costly radio frequency (RF) power. The necessity for cooling represents a more serious drawback for such devices than for the previously described DC arc device because it results in the loss of costly RF power. In the present invention, a cylindrical chamber is provided having a conical discharge vessel fabricated from a non-conducting temperature-resistant material. The interior of the cone communicates with the atmosphere or an exhaust system when operating at less than atmospheric pressure. The gas coolant, which for example may comprise argon, nitrogen, or air is supplied at high pressure from the chamber into the cone interior through a large number of capillary channels in the cone wall. After passing through the channels the gas is electrically ionized and accelerated towards the exit plane by an RF induction coil which is wound around the exterior of the cone Within the chamber.

The present invention overcomes the major disadvantage of prior designs by having the gas enter the discharge zone normal to the walls at velocities in excess of, or comparable to, the ion diffusion velocity, thus, the ions heat transfer by conduction to the walls is eliminated or sharply reduced. This raises the limit on practical power input. The gas itself acts as a coolant and carries thermal energy as well as charged particles back into the discharge zone. This results in an increased efiiciency of the torch. The cone geometry aids in expelling the discharge plasma by electromagnetic forces which become effective at higher power densities. Since the coil is enclosed in a highpressure atmosphere, the tendency to are between coil turns is reduced, permitting a higher concentration of RF power than in prior devices.

It is therefore an object of the invention to provide a novel and improved gas-cooled, electrodeless, plasma generator which uses RF power to continually ionize a gas and create a high-velocity, high-temperature gaseous discharge.

Another object of the invention is to provide a novel and improved method and apparatus for the generation of a plasma jet with increased efficiency.

It is another object of the invention to provide a novel transpiration cooled induction plasma jet torch.

Yet another object of the invention is to provide a novel and improved induction plasma torch which obviates water cooling or the like.

Still another object of the invention is to provide a novel and improved induction plasma torch having reduced losses as compared with prior devices of this general type.

Having in mind the defects of prior art plasma torches, it is an object of this invention to provide a transpiration cooled induction torch which overcomes the shortcomings of generally similar devices proposed heretofore.

These and other objects of the invention will be understood more completely from the following detailed description, taken in conjunction with the drawings, in which:

FIGURE 1 is a crosssectional View of a typical prior art device useful in the exposition of the present invention;

FIGURE 2 is a side elevation, cross-section view of a plasma generator constructed in accordance with the invention;

FIGURE 3 is a front elevation view of the apparatus of FIGURE 2; and

FIGURE 4 is a fragmentary cross-sectional view of the transpiration cone portion of the invention.

Looking first at FIGURE 1 there is shown an induction plasma torch typical of the prior art. This apparatus comprises a discharge vessel 1 having an inlet 2 which is connected to a source of pressurized gas. The vessel 1 is provided with a cooling jacket 3 having an inlet 4 for receiving water or other suitable coolant. The coolant leaves the jacket 3 via outlet 5. The cooling jacket 3 is provided with suitable baflle means 6 for properly distributing the water or other coolant around the vessel 1. The plasma flame, indicated generally at 7, is ejected from the open end of vessel 1. The gas is ionized by RF power applied to coil 10, which encircles the nozzle end of vessel 1. Leads 8 and 9 connect coil 10 to a suitable RF power supply. In this device a stationary high frequency discharge or plasma flame 7 is produced at medium pressure and the ionization level is limited mainly by the ambipolar diffusion of ions and electrons to the interior walls of the discharge vessel 1. The ionizing electric field must compensate also for the loss of ions and electrons which are swept out of the discharge zone by the gas stream. The character of the discharge is not greatly altered by this additional loss process even if the stream is supersonic. The reason is that most of the ionization is produced in the gas layer adjacent to the wall because here the induced electric field is highest. Also, this layer at the same time constitutes a highly viscous, and therefore slowly moving, fluid boundary layer. Therefore, most of the ionized particles are still captured by the wall and only the small fraction which are diffused into the free stream are swept out to form the ionization level of the plasma flame 7. Any attempt to raise this level by raising the power input is thwarted by other practical limits. For example, as the conductivity of the gas increases, the penetration depth of the applied field decreases which not only increases the difficulty of feeding power into the gas but also increases the wall losses disproportionately. Since the charged particles release their kineticand recombination energies to the Wall, the wall temperature rises strongly and requires cooling. Therefore, the power input and ionization level are ultimately limited by the heat flux that can be conducted through the tube material at structurally safe temperatures. Although quartz can withstand high temperature gradients it is a poor heat conductor.

There is shown in FIGURE 2 a typical construction of a plasma generator in accordance with the present invention which overcomes the above discussed shortcomings of prior devices. This embodiment comprises a cylindrical chamber 12 which is closed at one end by baffle or end plate 13 and which is closed at the other end by cone 14. End plate 13 is retained in place by groove 16, and is provided with an inlet conduit 15 through which a gas is applied under pressure, from any suitable source. Cone 14 is provided with an annular flange which is retained in groove 17. The cylindrical shape of chamber 12 can best be seen in FIGURE 3. Cone 14 is further provided with a plurality of capillary passages 18 which communicate the interior of chamber 12 with the atmosphere. A conical RF induction coil 19 is wound around the surface of cone 14 which extends into chamber 12. Coil 19 is connected to power leads 21 and 22. Highpressure gas, which for example may be argon or helium, entering through conduit 15 emerges through the various capillaries 18 in cone 14, generally in the direction indicated by arrow 23. After passing through the large number of capillary openings 18 in cone 14 the gas is electrically ionized by the RF energy supplied via coil 19, causing it to be heated and accelerated towards the exit plane which is normal to centerline 24. This generates the plasma flame indicated generally at 25.

As can be seen, the gas is supplied through the permeable walls of cone 14 so that it enters the chamber 12 radially rather than being fed into a discharge tube axially or tangentially as in prior art designs. This not only eliminates the need for a separate cooling fluid but it also reduces the need for cooling itself by reducing diffusion losses. The diffusion velocity of the ions cannot exceed the speed of sound in the surrounding medium; hence, if the gas enters uniformly at sonic or supersonic speeds, ions and electrons cannot reach the walls of cone 14 and can only be swept out into the flame region 25. This leaves a region 26 of relatively cool gas between the plasma flame 25 and the exposed interior surface of cone 14. The only heat transmitted to the walls therefore is by radiation. In view of the large pressure drops occurring in capillary channels it is difficult to obtain supersonic flow; however, the travel time of the ions to the walls is substantially increased by counterfiow of high subsonic velocity. If, for instance, of the ion diffusion velocity is obtained, the travel time is increased by a factor of five. Since, in this arrangement, the skin layer is continually traversed by a fresh supply of non-ionized gas, it can never become as thin as in a stationary or parallel flowing gas because the conductivity has to be first gen erated by the field. As a consequence, the RF field is enabled to transfer more energy to the gas. The ionized gas, on the other hand, is carried by the stream into the interior of the volume. This fluid dynamics process, therefore, overcomes the limitations set to the RF field by the penetration depth and also aids diffusion into the interior, so that a higher ionization level can be achieved.

In order to serve the intended purpose, the porosity of the capillary tubes 18 should be uniform and relatively high.

The development of a preferred construction can best be seen by first considering a transpiration nozzle comprising a cylindrical tube which has a porous section for transpiration of the gas and an exit section for the ejection of the plasma flame. As indicated, the porosity of the porous section should be uniform and relatively high. For example, a=25% where a is the sum of the cross sections of the individual capillaries divided by the total area of the cone A Assuming, for simplicity, critical values for velocity and density at the transpiration section a and p the rate of mass fiow entering the chamber is:

During steady state the same rate has to pass through the exit section of the tube A where again critical conditions are assumed a p which, because of the energy addition, will be different from those as Section 1. It is:

sin ,8 1

and with Equation 2:

MZ1*P1* fl BIG SID If no energy is added, a *=a and Equation 4 yields fi:l4.5 for ot=().25. With energy addition, 13

must be larger than 14.5" to prevent choking.

In addition to fabricating the discharge nozzle in a conical shape, the induction coil 19 is also fabricated in a conical shape. This aids ejection of the plasma by electromagnetic forces. The effect is significant only if RF magnetic fields in the coil are in the kilogaussrange which requires RF power in the megawatt range. Although such power levels are not readily available for continuous operation, it is contemplated to superimpose signal pulses from capacitor discharges onto a continuous Wave of practical power. Devices suitable for inductive generation and acceleration of plasmoids in vacuum by single pulses have been constructed heretofore.

Another advantage of the present invention results from the pre-ionization caused by the RF power which enhances the efficiency of energy transfer to the gas. Also, the invention permits the study of the behavior of plasmoids in the open atmosphere.

Depending on the power level, the invention will vary between an electrothermal and an electromagnetic propulsion device. In addition to propulsion studies, the apparatus of the invention provides a convenient means for simulating re-entry phenomena.

As can be seen in FIGURE 2, the cone 14 is inserted into a stagnation chamber with high enough over-pressure to overcome the pressure drop in the capillaries. This over-pressure will also prevent arcing between the coil turns, and permit RF power of the order of 100 kilowatts at 15 megahertz to be employed.

It will be understood that the drawing is illustrative of a typical embodiment of the invention, and that various changes and modifications maybe made without departure from its intended spirit and scope.

I claim:

1. A plasma generating apparatus comprising: conduit means;

means for effecting a flow of gas through said conduit means at a high pressure;

a conical member having a multiplicity of capillaries therethrough communicating said conduit means with the exterior of said apparatus, the apex of said conical member extending into said conduit means for radially discharging said gas into a zone exterior of said apparatus at velocities comparable to the diffusion velocity of the ions of said gas When ionized; and

induction means, located within said pressurized conduit means, which when energized will generate an electromagnetic field having an intensity sufliciently high to ionize gas in said zone and thereby generate a plasma flame which is ejected from said apparatus.

2. A plasma generating apparatus as defined in claim 1 wherein said induction means comprises:

a conical coil of complementary contour encircling said conical member, and located within said conduit means, whereby said high pressure gas serves to insulate the turns of said coil.

3. A plasma generating apparatus comprising:

conduit means;

transpiration means separating the interior of said conduit from the ambient atmosphere;

gas flow means for effecting a flow of gas at a velocity on the order of the local speed of sound in the gas through said conduit means and radially into said atmosphere via said transpiration means; and

coil means located within said conduit means for generating within said conduit means and adjacent said transpiration means a high intensity RF magnetic field having a substantial flux component transverse to the gas flow direction through said transpiration means, thereby inducing in said gas an electrical current flowing in a closed loop to generate a plasma flame which is spaced from said transpiration means and ejected into said atmosphere.

4. A plasma torch apparatus as defined in claim 3 in which said coil means is adapted to cause the direction of said magnetic field and the direction of said gas flow to be generally perpendicular to each other.

5. A transpiration cooled induction plasma generator comprising:

a body forming a chamber and having first and second apertures therein;

a conduit communicating with the interior of said chamber via said first aperture for the admission of highpressure gas into said chamber;

a porous non-conductive conical member, open at its base and closed at its apex, fabricated from a refractory material and mounted in said second aperture so that its apex is directed inwardly towards the interior of said chamber and its open base is directed toward the ambient environment;

an induction coil within said chamber encircling said conical element for establishing an RF flux field in the region of the interior of said conical member which is sufl'icient to generate a plasma.

6. A transpiration cooled induction plasma generator as defined in claim 5 wherein said conical member is provided with a plurality of individual capillaries, the sum of the cross sections of which exceeds 25% of the total area of said cone.

References Cited UNITED STATES PATENTS 2,919,370 12/1959 Giahnini 313-63 3,138,019 6/1964 Fonda-Bonardi -355 3,173,248 3/1965 Curtis 313-63 3,174,278 3/1965 Barger 313-63 3,214,623 10/1965 Sheer 313-63 FOREIGN PATENTS 938,133 10/ 1963 Great Britain.

JAMES W. LAWRENCE, Primary Examiner. STANLEY D. SCHLOSSER, Examiner. 

1. A PLASMA GENERATING APPARATUS COMPRISING: CONDUIT MEANS; MEANS FOR EFFECTING A FLOW OF GAS THROUGH SAID CONDUIT MEANS AT A HIGH PRESSURE; A CONICAL MEMBER HAVING A MULTIPLICITY OF CAPILLARIES THERETHROUGH COMMUNICATING SAID CONDUIT MEANS WITH THE EXTERIOR OF SAID APPARATUS, THE APEX OF SAID CONICAL MEMBER EXTENDING INTO SAID CONDUIT MEANS FOR RADIALLY DISCHARGING SAID GAS INTO A ZONE EXTERIOR OF SAID APPARATUS AT VELOCITIES COMPARABLE TO THE DIFFUSION VELOCITY OF THE IONS OF SAID GAS WHEN IONIZED; AND INDUCTION MEANS, LOCATED WITHIN SAID PRESSURIZED CONDUIT MEANS, WHICH WHEN ENERGIZED WILL GENERATE AN ELECTROMAGNETIC FIELD HAVING AN INTENSITY SUFFICIENTLY HIGH TO IONIZE GAS IN SAID ZONE AND THEREBY GENERATE A PLASMA FLAME WHICH IS EJECTED FROM SAID APPARATUS. 